1. Field of the Invention
The present invention relates to a polarization beam splitting element that reflects or transmits an entering beam according to its polarization direction, and particularly to a polarization beam splitting element that is constituted by a wire grid (one-dimensional grating structure) having a grating period smaller than a wavelength of the entering beam and by a dielectric multi-layered film layer.
2. Description of the Related Art
A wire grid that is a one-dimensional grating structure formed of a metal and having a grating period smaller than a wavelength (also referred to as a “use wavelength”) of an entering beam has a function of splitting beams whose polarization directions are mutually different. Japanese Patent Laid-Open No. 2003-502708 discloses a wire grid having a visible wavelength-order minute grating period and formed on a substrate. Moreover, Japanese Patent Laid-Open No. 2003-519818 discloses a wire grid polarization beam splitting element that is used by being sandwiched between transparent substrates such as prisms, for the purpose of suppressing generation of astigmatism at the substrate and of improving handling easiness of grating.
On the other hand, a dielectric grid that is a one-dimensional grating structure formed of a dielectric material and having a grating period smaller than the use wavelength has mutually different effective refractive indices in a grating longitudinal direction (direction orthogonal to a grating period direction) and in a grating width direction (corresponding to the grating period direction). Japanese Patent Laid-Open No. 2005-321562 discloses a polarization beam splitting element in which dielectric grids being laminated such that their grating period directions are orthogonal to one another are sandwiched by two prisms, for the purpose of utilizing the above-mentioned characteristic of the dielectric grid.
The wire grid reflects polarized light (hereinafter referred to as “first polarized light”) whose electric field oscillates in a direction parallel to the grid longitudinal direction, and transmits polarized light (hereinafter referred to as “second polarized light”) whose electric field oscillates in a direction orthogonal to the grid longitudinal direction (that is, orthogonal to the polarization direction of the first polarized light). However, in fact, the wire grid transmits or absorbs part of the first polarized light to be reflected, and reflects or absorbs part of the second polarized light to be transmitted. Conventional wire grids have an increased grating thickness so as to sufficiently split the first polarized light and the second polarized light (that is, so as to suppress the transmission of the first polarized light). However, the increased grating thickness increases the absorption of the second polarized light to be transmitted, which decreases transmittance therefore.
In addition, the wire grid polarization beam splitting element used by being sandwiched between the transparent substrates such as the prisms has a tendency to deteriorate characteristics for the second polarized light to be transmitted, which increases reflectance for the second polarized light as compared with a case where an entrance side medium is formed by air or formed as a vacuum space.
On the other hand, the polarization beam splitting element disclosed in Japanese Patent Laid-Open No. 2005-321562 which uses the dielectric grids utilizes total reflection for s-polarized light reflection, so that reflectance for s-polarized light with small incident angles significantly decreases.